Achieving accurate, real-time bottom hole pressure measurements during borehole stimulation treatments has long been a goal in the oil and gas industry. During fracture treatments, in particular, accurate measurement of bottom hole pressure would allow an operator to observe fracture growth trends in real-time, and change treatment conditions accordingly. However, real-time measurements of bottom hole pressure are rarely performed with current technology because the abrasiveness of a fracturing slurry is destructive to any exposed cable placed in the wellbore for delivering data to the surface. Downhole memory gauges are sometimes used for selected treatments, but these do not enable real-time decision making during the treatment because their data is not delivered to the surface until after the treatment is over.
One attempt to deliver bottom hole pressure measurement data in real-time is described in Doublet, L. E., Nevans, J. W., Fisher, M. K., Heine, R. L, Blasingame, T. A., Pressure Transient Data Acquisition and Analysis Using Real Time Electromagnetic Telemetry, SPE 35161, March 1996 (“Doublet”). Doublet teaches that pressure measurements are transmitted from a downhole gauge to the surface through the formation strata via electromagnetic signals. Although this technique has been used successfully on some wells, it is limited by the borehole depth and the types of rock layers through which a signal could be transmitted clearly. In particular, electromagnetic signals are rapidly attenuated by the formation. These limitations render the technique impractical for use in many wells, and particularly in deep wells.
It is known that implosions at depth in a fluid filled borehole are effective seismic sources. For example, imploding spheres and other shapes have been used as underwater acoustic sources for ocean applications as described in Heard, G. J., McDonald, M., Chapman, N. R., Jashke, L., “Underwater light bulb implosions—a useful acoustic source,” Proc IEEE Oceans '97; M. Orr and M. Schoenberg, “Acoustic signatures from deep water implosions of spherical cavities,” J. Acoustic Society Am., 59, 1155-1159, 1976; R. J. Urick, “Implosions as Sources of Underwater Sound,” J. Acoustic Society Am, 35, 2026-2027, 1963; and Giotto, A., and Penrose, J. D., “Investigating the acoustic properties of the underwater implosions of light globes and evacuated spheres,” Australian Acoustical Society Conference, Nov. 15-17, 2000. Typically, a device with a vacuum or low pressure chamber is released into the water to sink and eventually implode when the hydrostatic pressure exceeds implosion threshold of the device. A triggering mechanism may be used to cause the device to implode before pressure alone would do so as described in Harben, P. E., Boro, C., Dorman, Pulli, J., 2000, “Use of imploding spheres: an Alternative to Explosives as Acoustic Sources at mid-Latitude SOFAR Channel Depths,” Lawrence Livermore National Laboratory Report, UCRL-ID-139032. One example of an implosive device is commercial light bulbs, as described in both Heard, G. J., McDonald, M., Chapman, N. R., Jashke, L., “Underwater light bulb implosions—a useful acoustic source,” Proc IEEE Oceans '97; and Giotto. The controlled use of implosive sources in a wellbore is described in U.S. Pat. No. 4,805,726 of Taylor, D. T., Brooks, J. E., titled “Controlled Implosive Downhole Seismic Source.” Seismic sources generate low frequency tubewaves which propagate up and down the borehole over long distances with a clearly defined velocity and little dispersion, particularly in cased wells. Indeed, tubewaves propagate with so little attenuation that they are the major source of noise in conventional borehole seismic surveys. Tubewaves are described, for example, in White, J. E., 1983, “Underground Sound: Application of Seismic Waves,” Elsevier, ISBN 0-444-42139-4 (“White”).